KR100208299B1 - Method and apparatus for removing sulfur dioxide from gas streams - Google Patents

Abstract

The present invention relates to a method and apparatus for removing SO ₂ from a low volume gas stream comprising a high concentration of SO ₂ , comprising a tank scrubber comprising an aqueous SO ₂ adsorbent and an impeller. The impeller includes a vertical axis of rotation with upper and lower impellers. The lower impeller acts to agitate the SO ₂ adsorbent. The upper impeller includes a lid to guide the adsorbent and the gas outwardly with respect to the wall of the scrubber. Gas is injected into the adsorbent at the bottom of the covered impeller. The lid promotes gas-liquid mixing by preventing gas from passing through, resulting in a significant improvement in SO ₂ removal efficiency.

Description

How to remove sulfur dioxide from a gas stream

1 is a schematic diagram of an apparatus for carrying out a process in a preferred embodiment of the invention.

2 is a bottom perspective view of a shrouded impeller in accordance with a preferred form of the present invention.

The present invention relates to a method and apparatus for cleaning sulfur dioxide from an SO ₂ containing gas (hereinafter referred to as flue gas or simply gas). In particular, the present invention relates to a dispersed gas phase sulfur dioxide scrubber or tank scrubber provided with an impeller device for dispersing SO ₂ containing gas in an aqueous absorbent.

Large quantities of sulfur dioxide are released from plants around the world each year, and strong laws are in place in most countries to regulate these emissions. Wet cleaning systems [eg; Spray towers are the most common means used to remove sulfur dioxide from flue gases. Wet scrubbers typically use alkaline reagents [eg, to remove SO ₂ from flue gas]; Aqueous solution of sodium bicarbonate, sodium carbonate, lime or limestone or an aqueous slurry. Such solutions and slurries of alkaline reagents are often referred to as SO ₂ absorbers or simply absorbers. Conventional wet scrubbers have proved to be very effective means of removing SO ₂ , but they tend to be very expensive to maintain due to massive scale deposition. In addition, the SO ₂ removal efficiency of conventional wet scrubbers is only about 90%, which is not always adequate to ensure that the emitted flue gas meets regulatory standards in numerous regions around the world.

Tank scrubbers replace conventional wet scrubbers. Tank scrubbers generally provide a vessel comprising an aqueous slurry or aqueous solution of alkaline reagents (usually lime or limestone) that act as absorbents for SO ₂ . Flue gas is injected directly into the absorbent slurry. Several devices have been proposed for contacting the SO ₂ containing gas with the absorbent slurry. For example, in US Pat. No. 4,099,925, flue gas is contacted with an absorbing slurry in a liquid-raising pipe. In US Pat. No. 4,156,712, flue gas is sparged on top of the liquid absorbent above flue gas injection without the aid of certain mechanical agitation. In US Pat. No. 4,229,417, flue gas dispersion is achieved in a manner similar to that of US Pat. No. 4,156,712, except that the pipes supplying the flue gas into the sorbent slurry have a notch provided for better gas dispersion. do. US Patent No. 4,911,901 describes a process in which flue gas is contacted with a spray of sorbent slurry prior to gas-slurry contact in the reactor. The SO ₂ removal efficiencies obtained by such prior art tank scrubbers range from 90 to 90%, which is a significant improvement over conventional wet scrubber efficiencies. The efficiency of these tank scrubbers is not always suitable to meet statutory requirements, especially when the SO ₂ concentration in the flue gas is relatively high, more particularly when the gas comprises more than 1% SO ₂ .

Generally, tank scrubbers include a device for bubbling flue gas through an absorbent. As a result, tank scrubbers are typically limited to the volume of flue gas that can be treated.

Currently used tank scrubbers are limited due to the difficulty in handling very large volumes of flue gas, but are often used in tank scrubbers in plants that produce relatively small volumes (ie, less than 25,000 CMF) and produce SO ₂ content of flue gas. There is a potential application. For example, newly developed metallurgical processes (eg, oxygen-extended gold mine roasting) produce these gases. The Small Claus plant also produces this gas. Due to the high efficiency of tank scrubbers when used to treat low volume flue gases, the number of potential applications must be increased with increasing environmental protection legislation. Successive improvements in tank scrubber designs provide even more opportunities for the use of tank scrubbers.

Accordingly, there has been a continuing need to improve the efficiency of tank scrubbers for removing SO ₂ from flue gas and other gas streams. There is also a special need for an improved tank scrubber capable of removing large amounts of SO ₂ from a low volume stream of flue gas comprising high concentrations of SO ₂ .

Accordingly, the present invention relates to a method and apparatus for providing an improved tank scrubber design for removing SO ₂ from flue gas. The method and apparatus of the present invention are those capable of effectively removing SO ₂ from a low volume stream of flue gas containing high concentrations of SO ₂ .

The present invention relates to a method and apparatus for cleaning sulfur dioxide from a SO ₂ containing gas, in particular flue gas comprising sulfur dioxide in an amount greater than 1 vol%. SO ₂ and preferably SO ₂ containing gas containing O ₂ is compressed to a pressure of about 2 to 8psig is injected into the new SO ₂ absorber feed tank the washing machine. Suitable absorbents include, for example, aqueous solutions or aqueous slurries of sodium bicarbonate, sodium carbonate or sodium hydroxide and aqueous slurries of lime and / or limestone and mixtures thereof. Preferred absorbents are aqueous slurries of lime. The tank scrubber comprises stirring means for dispersing gas in the absorbent, circulating the aqueous absorbent and maintaining the solid in suspension. The stirring system of the tank scrubber comprises an upper impeller means designed to provide high shear forces and a lower impeller means designed to stir. The upper and lower impellers are preferably placed on a single rotating impeller axis located in the tank so that the impeller is present below the surface of the absorbent. In another aspect, the impeller may be located on a separate axis of rotation. In a preferred embodiment, the lower impeller is a pitched blade. Preferably, the upper impeller is a flat blade disc design having a circular disk that acts as a lid and a plurality of flat blades or wings extending radially terminating at the outer edge of the disk.

The flat blade is fixed under the disk. The cover of the lower impeller is preferably located between 1 and 3 ft below the surface of the absorbent solution or slurry. The SO ₂ containing gas is injected into the solution or slurry of the SO ₂ absorbent and dispersed in the solution or slurry by the upper impeller. The pH of the aqueous absorbent is preferably maintained at about 4.5 to 7.0 by adding fresh absorbent. The solution or slurry temperature is preferably maintained at 33 to 185 ° F. The SO ₂ containing gas is injected at a point near the impeller axis between the upper and lower impellers at 3-5 feet below the surface of the absorbent solution or slurry held in the tank scrubber. It is preferable to inject the SO ₂ containing gas downward using a single nozzle gas injection means. In addition, multiple injection nozzles may be used.

SO ₂ contained in the gas is dissolved in the aqueous phase of the absorbent and reacted with the absorbent to react intermediate products such as sodium bisulfite NaHSO, sodium sulfite Na ₂ SO ₃ , calcium bisulfite Ca (HSO ₃ ) ₂ and / or calcium sulfite CaSO ₃ ]. If enough oxygen is used, the corresponding sulfate is formed. If insufficient oxygen is used in the injection gas to complete the reaction to Na ₂ SO ₃ or CaSO ₄ , additional oxygen is added directly to the tank scrubber solution or slurry via a gas inlet line or a solution or slurry from the tank scrubber. It can be applied directly into the effluent line.

In one preferred embodiment of the invention, the process is carried out as a continuous process. The new aqueous absorbent is continuously introduced into the tank scrubber so that the absorbent in the tank comprises a mixture of fresh absorbent and used absorbent. The new absorbent is introduced into the tank at a rate that initiates injection of the SO ₂ containing gas so that the pH of the aqueous absorbent is preferably maintained at about 4.5 to 7.0 and more preferably at about 5.4 to 6.0.

The SO ₂ absorbent used is recovered from the stirred tank at a rate sufficient to maintain a constant amount of solution or slurry above about 1 to 3 feet of the covered impeller.

The disadvantages and limitations of the prior art tank scrubbers are eliminated by the method and apparatus of the present invention. The present invention relates to a method of removing sulfur dioxide from a gas, in particular flue gas, using a novel tank scrubber. The process of the present invention can effectively remove large amounts of sulfur dioxide from a gas comprising from 1 to 50 volume percent sulfur dioxide.

SO ₂ containing gases (eg flue gases) are injected into tank scrubbers containing SO ₂ absorbents. The tank scrubber comprises impeller means including a capped high shear upper impeller. The impeller means further comprise a pitched blade lower impeller which acts to agitate the aqueous absorbent. The upper and lower impellers are preferably located on a common vertical axis of rotation with upper and lower regions for positioning the impellers. The impeller may be located on a separate axis. The SO ₂ containing gas is preferably injected into the aqueous absorbent at a point between the upper and lower impellers of about 3 to 5 feet below the location of the aqueous absorbent. The upper impeller provides shearing action to disperse the injected gas and accelerate the absorption of SO _{2 into} the absorbent. The cover on the upper impeller substantially prevents the SO ₂ containing gas from bubbling from the absorbent before it is dispersed in the absorbent. The blades of the lower impeller may provide agitation action and may be pitched to elevate the absorbent along the axis or to otherwise propel the absorbent to the bottom of the tank.

In a preferred embodiment of the invention, the SO ₂ absorbent is an aqueous solution or aqueous slurry of alkaline absorbent material. Preferred alkaline absorbent materials are calcium carbonate [eg limestone], calcium oxide [eg lime] and calcium hydroxide [eg slaked lime]. Other satisfactory alkaline absorbent materials may include, for example, sodium bicarbonate, sodium carbonate and sodium hydroxide.

The process of the invention is preferably a continuous process in which fresh aqueous absorbents are added continuously to the reaction vessel and the absorbents used are continuously removed. Although less preferred, the process can be carried out in a batch process. The SO ₂ containing gas is injected into the absorbent as described in more detail below. Throughout the injection of SO ₂ containing gas and the addition of fresh absorbent, the pH of the aqueous absorbent in the tank scrubber is preferably maintained at about 4.5 to 7.0, more preferably at about 5.4 to 6.0. Under certain operating conditions, the pH of the absorbent may be raised above 7.0. The pH of the aqueous absorbent is preferably adjusted by adjusting the rate of fresh sorbent addition and gas injection so that the pH is maintained at the desired level. The amount and rate of addition of the new absorbent depends on the concentration of the absorbent and the sulfur dioxide content of the gas to be treated.

An apparatus according to a preferred embodiment of the invention is shown in FIG. The tank scrubber 4 is a top closed vessel equipped with a stirring system consisting of an upper impeller 6 with a lid for providing high shear forces and a pitched blade lower impeller 8 for providing agitation. Preferably, the covered impeller 6 is attached with four or six flat rectangular blades 20 for gas dispersion. As shown in FIG. 1 the tip of the blade coincides with the outer edge of the impeller cover 7. The size of the impeller will change depending on the size of the tank and other factors. For example, a 20ft diameter tank typically requires an upper impeller of about 6 to 7ft diameter and a lower impeller of about 9 to 10ft diameter to achieve maximum efficiency. The lid 7 prevents the passage of the injected gas along the impeller shaft 5 and allows the gas to be injected at a relatively shallow depth in the aqueous absorbent without disturbing the flue gas residence time in the absorbent. When the gas injection is made shallow, the horsepower requirement during the gas injection is reduced. The upper and lower impellers 6 and 8 are preferably attached to the common impeller shaft 5 which is driven by the motor 9.

Tank scrubber solution or slurry 10 (hereinafter referred to as tank scrubber fluid or simply fluid) held in tank scrubber 4 comprises an aqueous mixture of fresh SO ₂ absorbent and used SO ₂ absorbent. The tank scrubber fluid 10 maintains the pH above 4.5, preferably above 5.4 by adding fresh absorbent. When using an aqueous slurry of lime as a fresh absorbent, the tank scrubber fluid 10 maintains the pH preferably in the range of 5.4 to 6.0, preferably 4.5 to 7.0. Additional new SO ₂ absorbent is added continuously through line 13 to adjust the pH of the tank scrubber fluid 10. Although the pH of the tank scrubber fluid can be raised to above 7.0, reactions occurring between Ca (OH) ₂ and carbon dioxide contained in the flue gas at pH values above 7.0 increase CaCO ₃ formation, increase scale deposition and lime Causes a tendency to increase consumption. At pH values below 4.5, the SO ₂ solubility is too low to have a decisive impact on scrubber efficiency. The tank scrubber fluid surface level 11 is preferably held at about 1 to 3 feet above the covered impeller 6. The scrubber fluid surface level 11 is controlled by continuously or periodically recovering the slurry contained in the tank scrubber 4. The tank scrubber 4 is designed to provide a gas recovery space 12 on top of the absorbent fluid to recover the treated gas prior to exiting the scrubber 14 through line 14.

In FIG. 1, flue gas comprising SO ₂ and preferably O ₂ and compressed to about 2-8 psig is fed to tank scrubber 4 via lines 1 and 2 and injection nozzle 3. As shown in FIG. 1, the flue gas injection nozzle 3 preferably injects flue gas downward in a direction towards the lower impeller 8. Flue gas is injected below the surface of the tank scrubber fluid 10 at a point near the stirrer shaft 5 just below the covered upper impeller 6. The flue gas injection point is preferably about 3 to 5 feet below the tank scrubber fluid surface level 11. Preferably, a single injection nozzle 3 is used to avoid the potential scale deposits typically created by using multiple injection points. The interior of the injection nozzle 3 can be kept clean by the action of a number of water droplets (not shown) towards the inner nozzle lip and the inner nozzle pipe surface.

SO ₂ in the flue gas reacts with the absorbent contained in the tank scrubber fluid 10 to react intermediate products such as sodium bisulfite NaHSO ₃ , sodium sulfite Na ₂ SO ₃ , calcium bisulfite Ca (HSO ₃ ) ₂ and / or Calcium sulfite CaSO ₃ ]. These intermediate products further react with oxygen to form the corresponding sulfate when sufficient oxygen is present in the scrubber. If there is insufficient O ₂ available in the flue gas (ie less than about 5 vol% O ₂ ) to effectively complete the oxidation of the intermediate product, an O ₂ source can be added. In a preferred embodiment of the present invention, the flue gas comprises at least about 5 volume percent oxygen when injected into the absorbent. This additional O ₂ source can be added to the flue gas inlet line 1 using line 18. An additional source of O ₂ is provided with a dispensing system such as a nozzle 19 which can be positioned below the surface 11 of the tank scrubber fluid 10 adjacent the flue gas injection nozzle 3 and present under the upper impeller 6. Through the tank scrubber fluid 10 directly. In another embodiment, the effluent tank scrubber fluid 10 exiting the tank scrubber 4 by the lines 15 and 17 and the valve 16 is by means (not shown) external to the tank scrubber 4. Treatment may be with air or oxygen. Oxygen added to the flue gas to replenish the oxygen content of the gas is generally in the form of an oxygen containing gas [eg air], although oxygen may be used in a more pure or concentrated form.

The tank scrubber 4 is designed to be sized to hold the fluid for at least 30 minutes, preferably 8 to 24 hours. Tank scrubber fluid 10, maintained at 33 to 185 ° F., is withdrawn from scrubber 4 through line 15, control valve 16, and line 17. When using sodium bicarbonate or sodium carbonate solution as absorbent, the obtained sodium sulfate solution can be regenerated using lime as is known in the art. As in the preferred embodiment, when lime is used as the absorbent, the tank scrubber fluid 10 is sent to a tailings pond for disposal or the fluid is dehydrated to obtain its solids for use in industrial applications. It can be recovered.

A unique advantage of the tank scrubber 4 of the present invention is the stirring system. By preventing short circuiting of the gas on top of the stirrer shaft 5, the top cover 7 allows relatively shallow injection of flue gas and still allows good gas-fluid contact in the minimum volume of absorbent. This minimizes the need for flue gas compression (ie, about 2-8 psig) and significantly reduces horsepower. The lid 7 in combination with the two Ipeller means provides vigorous agitation and mixing of the gas and tank scrubber fluid 10 and consequently promotes fast and effective absorption of SO ₂ by the tank scrubber fluid 10. .

2 is a bottom perspective view of the covered impeller 6 and the lid 7. In this embodiment, the impeller has six flat rectangular blades 20 situated just below the lid 7. As shown, the cover 7 is a flat disk-shaped member that is substantially fixed to the impeller shaft 5. The blade 20 is substantially rectangular in shape fixed to the lower side of the lid 7. The blade 20 is preferably located in a vertical plane substantially parallel to the axis of the impeller shaft 5. In the embodiment shown in FIG. 2, the impeller blades extend radially outward from the impeller shaft 5 and terminate at the outer edge of the lid 7. Preferably, the blades 20 are slightly spaced apart on the axis 5. Alternatively, the blade 20 contacts the shaft 5 and extends radially outward with respect to the outer edge of the lid 7. In a further aspect, the impeller blades 20 can be pitched about the axis of the impeller shaft 5.

The lid 7 and the impeller blades 20 are preferably dimensioned to prevent the injected flue gas from passing upward along the impeller shaft and continually exiting the scrubber. The axis 5 is rotated at a speed sufficient to guide the flue gas radially outwardly and to disperse the flue gas in the absorbent to promote sufficient retention time to effectively remove sulfur dioxide.

An important advantage of the present invention is the efficiency of removing SO ₂ from the flue gas. Prior art tank scrubbers provide SO ₂ removal efficiencies in the range of 90 to 99%, and more typically SO ₂ removal efficiencies for flue gases containing relatively small amounts of sulfur dioxide. In contrast, the apparatus and process of the present invention provide a 99.9% grade SO ₂ removal efficiency based on full scale plant operation data, which is a size grade higher than the highest obtained in the prior art. The method and apparatus of the present invention have been found to be effective in removing sulfur dioxide from flue gases containing large amounts of sulfur dioxide as well as other contaminants.

Another advantage is the simplicity of the design. For example, flue gas is injected into the tank scrubber fluid 10 through a single injection nozzle 3 while the prior art methods typically use multiple injection pipes. Based on experience derived from full scale plant operation, the simplicity of the design means less maintenance costs and ease of operation. The ability of the device to use a single injection nozzle rather than using multiple nozzles is due to the efficiency of the covered impeller to effectively disperse the flue gas in the absorbent.

Another obvious advantage of the present invention is to provide a mechanical method of producing gas dispersion. The method of the present invention effectively removes SO ₂ from the flue gas by using a capped impeller 6 which produces microdispersion of gas in the tank scrubber fluid 10.

Still another advantage of the present invention is that scale formation on tank apparatus is largely eliminated in systems using calcium compounds such as lime as SO ₂ absorbent. In an embodiment of the present invention, calcium sulfite crystals are achieved due to the tendency to grow predominantly on undispersed solids produced by the innovative techniques used. Other advantages of the present invention will be apparent to those skilled in the art.

In one preferred embodiment, a single tank is used. A second support tank or scrubber tank can be used in conjunction with the first tank to increase retention time to allow for proper growth.

The system is particularly suitable for treating off-gases from gold mines, oxygen-extended roasting processes as described in US Pat. No. 4,919,715. Flue gas of about 2600 SCFM with an SO ₂ content of 4-14% and an O ₂ content of about 15-17% is released from the gold mine roasting process. The operating efficiency of the tank scrubber unit during full scale roasting plant operation is above 99.9%. The novel tank scrubbers of the present invention are suitable for other oxygen-enhanced gold mine roasting plants and also for similarly sized plants in other industries, in particular plants which emit relatively small volumes of high SO ₂ containing flue gas (e.g. Klaus plants). Do. As described above, the oxygen providing gas can be injected separately as necessary, but it is also a benefit that a substantial amount of O ₂ is present in the flue gas.

EXAMPLE

Flue gas of 2660 SCFM stream from a gold mine oxygen roasting plant containing 4% SO ₂ and 17% O ₂ is fed into a tank scrubber as shown in FIG. 1. The flue gas temperature and pressure are 160 ° F and 3.8 psig, respectively. The tank scrubber is 20ft in diameter and 22ft in height. The tank scrubber is equipped with a cover 82 having a diameter of 82 inches and a covered impeller with six blades, each 38 inches long and 12.2 inches high, welded to the lid so that the outer end of the blade coincides with the outer edge of the lid. It is. Place a pitched blade impeller 112 inches in diameter on the same axis under the covered impeller. The relative positions of the impellers are about 144 inches and 37 inches from the bottom of the tank, respectively. The impeller is equipped with a 75 horsepower motor and runs at about 37 RPM. The tank contains 20% by weight calcium sulfate slurry, the pH of which is maintained in the range of 5.4 to 6.0 by continuously adding lime slurry. The amount of slurry in the tank scrubber is adjusted to about 15 ft. Excess slurry is continuously withdrawn from the tank scrubber to maintain a constant amount of slurry. The gas stream exiting the scrubber contains about 20 ppm SO ₂ , which corresponds to about 99.95% SO ₂ removal efficiency.

Claims (25)

a. Adding an aqueous SO ₂ absorbent into the vessel in an amount such that the pH of the SO ₂ absorbent fluid is maintained above 4.5, b. Stirring the aqueous SO ₂ absorbent fluid using a rotating impeller comprising an upper impeller and a lower impeller, wherein the upper impeller includes one or more shrouds to inhibit gas passage; c. Injecting a SO ₂ containing gas below the surface of the aqueous absorbent fluid in the vicinity between the upper and lower impellers, d. Maintaining the level of the aqueous absorbent fluid in the vessel at a predetermined height by recovering the aqueous absorbent fluid; and e. Including the step of removing desulfurized gas from the container how to remove SO ₂ from the SO ₂ containing gas. The process of claim 1 wherein the SO ₂ absorbent fluid is an aqueous solution or aqueous slurry of alkaline components selected from the group consisting of sodium bicarbonate, sodium carbonate, sodium hydroxide, slaked lime, limestone and mixtures thereof. The method of claim 1 wherein the SO ₂ content of the gas supplied to the vessel is from about 1 to about 50 volume percent. The method of claim 1 wherein the gas supplied to the vessel comprises oxygen and its oxygen content is at least about 5% by volume. The method of claim 1, further comprising providing at least about 5% by volume of total oxygen content in the gas by applying oxygen to the gas prior to injecting the absorbent fluid. The method of claim 1, further comprising adding an oxygen containing source in the absorbent fluid in an amount sufficient to oxidize substantially all of the sulfite and bisulfite products in the aqueous absorbent fluid to sulfates. The method of claim 1, further comprising injecting oxygen into the aqueous absorbent fluid recovered from the vessel of step (d) to oxidize substantially all of the sulfite and bisulfite products to sulfates. The method of claim 1, further comprising compressing the gas to about 2 to 8 psig prior to injecting the gas into the aqueous absorbent fluid. The method of claim 1, wherein the temperature of the fluid is maintained at about 33 to 185 ° F. The method of claim 1 wherein the SO ₂ absorbent fluid is a slurry of lime. The method of claim 1 wherein the pH of the aqueous absorbent fluid in the vessel is maintained at about 5.4 to 6.0. The method of claim 1, further comprising maintaining the aqueous absorbent fluid in the container for at least 30 minutes. The method of claim 1, further comprising maintaining the aqueous absorbent fluid in the vessel for about 8 to 24 hours. The method of claim 1, further comprising maintaining a level of the aqueous absorbent fluid above about 1 to 3 feet of the upper impeller. The method of claim 1, further comprising injecting a gas about 3 to 5 feet below the surface of the absorbent fluid. The method of claim 1, further comprising injecting gas into the aqueous absorbent fluid through a single nozzle. The method of claim 1 wherein the upper impeller and the lower impeller are located on a common axis of rotation. The method of claim 1 wherein the upper impeller and the lower impeller are each located on separate axes of rotation. a. The upper and lower regions, wherein the lower region has a pitched blade first impeller means for agitating the aqueous SO ₂ absorbent, the upper region of the shaft comprising a second impeller means attached to the top of the first impeller means and fixed to the shaft A stirrer means comprising a substantially vertical axis of rotation having a substantially horizontal cover, and means for guiding the aqueous absorbent substantially radially from the axis, wherein the container comprises an aqueous SO ₂ absorbent, wherein the container The pH of the aqueous absorbent in the genus is at least 4.5], continuously introducing the SO ₂ containing gas; b. The SO ₂ containing gas is injected into the aqueous absorbent at the point of attachment between the upper impeller means and the lower impeller means, and the impeller shaft is rotated at a rate that produces sufficient shear force, thereby simultaneously stirring the aqueous absorbent to radially inject the flue gas injected outward Dispersing the gas in the aqueous absorbent by guiding it to absorb SO ₂ from the gas and c. Continuous process for removing the SO ₂ from a step of removing the gas injected from the aqueous absorbent, SO ₂ containing gas. 20. The method of claim 19, further comprising continuously introducing the aqueous absorbent into the vessel at a rate such that the pH of the aqueous absorbent in the vessel is at least 4.5. 20. The method of claim 19, further comprising maintaining a substantially constant amount of the absorbent by continuously removing the used aqueous absorbent from the vessel. The method of claim 1, further comprising rotating the upper impeller at a rate that provides sufficient shear force to disperse the SO ₂ containing gas. The method of claim 1 wherein the lower impeller comprises a pitched blade impeller for stirring the aqueous absorbent. 24. The method of claim 23, wherein the upper impeller comprises a substantially horizontal circular disk fixed to a substantially vertical axis of rotation and a plurality of wings attached to the lower side of the disk and extending radially from the axis to the outer edge of the disk. The method of claim 24, further comprising injecting an oxygen containing gas near an axis located between the upper and lower impellers.